Top seed Serena Williams survived an unexpectedly tough examination of her title credentials before reaching her sixth Wimbledon final.Vera Zvonareva stands between Williams and a fourth title after the world number one battled past Petra Kvitova 7-6 (7-5) 6-2 on Thursday.Kvitova, in her first Grand Slam semi-final, became the first player to break Williams' serve at Wimbledon.But she let slip a first-set lead and the top seed's experience told.After a tussle lasting one hour and 32 minutes, Williams celebrated with a curtsey for the crowd."It wasn't easy, I worked really hard," she told BBC Sport."I didn't expect to get this far the way I started at the beginning of the tournament. I just felt off, but I'm happy to still be here. It's such a blessing to just be in the tournament."Williams' route to a fourth title looked relatively simple after her sister Venus and Kim Clijsters were beaten in the quarter-finals.Kvitova had never won a match on grass before arriving at Wimbledon this year, and the 20-year-old admitted before her debut Grand Slam semi-final that she did not think she could beat Williams.But her performance on Centre Court, certainly in the early stages, belied that statement, a magnificent low volley setting up break point at 2-2 before Williams netted a forehand to find her serve broken for the first time in the tournament.Kvitova even had a point for a 5-3 lead, but three forehand errors allowed Williams to get back on level terms.Still, the unseeded Czech refused to roll over completely, fighting back from 4-0 down in the tie-break and saving two set points before Williams delivered a crunching first serve to take the first set by the narrowest in just under an hour.Williams, whose 80 aces at the tournament puts her streets ahead of her female rivals and would put her seventh on the men's list, found the second set a little easier to negotiate, but Kvitova still provided flashes of brilliance for an appreciative crowd.Trailing 4-2 and break point down, the pair played out a thrilling 19-shot rally, Kvitova somehow scrambling back a lob before finally firing a forehand past an exhausted Williams.The effort, though, was apparently too much for the 20-year-old and she double-faulted on the next break point to allow Williams to come through.Source: BBC